# Another Dumb Fin Question !

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#### cdma77

##### Well-Known Member
I was wondering why everthing is a trapazoid for fin shapes. I am planning on having a 3 inch diamter frame, 67" total length with nose cone. My total weight would be about 4.6 lbs with parachute and electronics. The motor should add about another 1.5 lbs. I was planning on using 6"x6" right angle triangle fins. I tried to understand Rocsim but some of the paramters are confusing me. Does anybody have any idea of this will be stable? I had a formula that I found that said to calculate fin size to take .17*(d+.5)*L. Based on that my fin size would be 9"x9" which sounds crazy ! Please help me out !

Thanks,

Jeff

Sorry, but I don't want to post what I did in your other thread twice.

I'll leave it at this: some shapes are more efficient in maximizing their effect on the center of pressure of a rocket than others for a given fin area. Some shapes provide less drag for a given fin area. Trapezoidal is probably an amalgamation of the two.

Trapezoidal might just be popular or was easier to build in full size "real-world" rockets, and therefore carried over as a base shape in model rockets as the models were made to somewhat copy non-model rockets.

Personally, I have no way of answering your question if its in regard to performance and drag - for that you need to use a simulation program (RockSim being very popular.)

If you really want to use your right triangle fins, use them. You can make them work no matter what. They are certainly not too small for a rocket that size.

I'm also not sure if there is a concrete answer

you want a fin large enough to provide ample restoring force
alot of it depends on length of sustainer in relation to the diameter and weight(particulary noseweight)

mabey design your fins first , than play with it in rocsim

I read somewhere that a slightly rearward slanted rectangular fin( one short side being the root edge) is the most efficient shape

I'm sure others will help verify(or debunk me....lol.)

ofcourse, near and above mach ,ideal shape/size can change.... as well

.ok I just noticed the other post...looks like your fin design should be fine

Why trapizoid fins?

They look cool (personal opinion...) I use them on the Praetor, Richter Recker, Cheetah, Deuce's Wild!... guess i'm kinda stuck on them

Also, they are VERY easy to cut out... a must when you cut out as many fins as I do in a typical year... LOL

Originally posted by cdma77
I was wondering why everthing is a trapazoid for fin shapes. I am planning on having a 3 inch diamter frame, 67" total length with nose cone. My total weight would be about 4.6 lbs with parachute and electronics. The motor should add about another 1.5 lbs. I was planning on using 6"x6" right angle triangle fins. I tried to understand Rocsim but some of the paramters are confusing me. Does anybody have any idea of this will be stable? I had a formula that I found that said to calculate fin size to take .17*(d+.5)*L. Based on that my fin size would be 9"x9" which sounds crazy ! Please help me out !

Thanks,

Jeff

All else being equal, it's because of drag. Seen from the top, the area presented to the oncoming air is larger the farther the fins stick out from the body. With triangular fins, the part that sticks out away from the body the most has relatively little control area (are that can be presented for stabilization when the bird starts to tilt). That is, at some point along the span (how much it sticks out) you go from gaining foremost in flight control to gaining foremost in cross-sectional area (ie. drag). If you figure out where that point of diminishing returns is, and cut the fin off there, guess what shape you have.

There's another issue that crops up in airplanes, but I haven't seen much on in rocketry, and that's fin tip vortices. Fin/wing tips cause a small horizontal tornado-like disturbance. This adds a fair amount to drag. Recently designers have added small wingtip fins like little rudders to the wings to reduce this. Airlanes need long wings, and so this is the best fix. Rockets don't, so clipping them makes more sense. Here's a bit about wing drag and vortices:

https://www.aero.ses.soton.ac.uk/courses/ME162/WebDBT/Demonstration.htm

As for stability testing, remember the cut-out method, included in this fine rocket stability primer paper:

www.rockets4schools.org/education/ Basic_Rocket_Stability.pdf

Jeff,

What version of RockSim do you have? I could put together a file to help you get started (even with the demo version). The attached RockSim version 7 file is the best that I can do based on what you said with the 6 x 6" fins. Looks like a great design, it gives stable flights on 54mm I-K motors if the weight is properly distributed.

Post more details and I will create a better simulation file for you if you want.

Bruce S. Levison, NAR #69055

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